1. Field of the Invention
The present invention provides aqueous preparations of isocyanate-reactive polymer dispersions having surface-deactivated oligomeric solid isocyanates dispersed in finely particulate manner and certain catalysts. The invention likewise provides the use of the preparations as latently reactive binders, for coatings as well as for adhesives in the form of layers, films or powders, as well as substrates which are coated therewith.
2. Description of the Prior Art
Aqueous preparations of isocyanate-reactive polymer dispersions having surface-deactivated, uretdione group-containing oligomeric solid isocyanates dispersed in finely particulate manner, as well as the use thereof as latently reactive binders for coatings as well as for adhesives are known.
EP-A 0 204 970 describes a process for the production of stable dispersions of finely particulate polyisocyanates by treating the polyisocyanates in a liquid with stabilisers and with the action of high shear forces or grinding. Di- and polyisocyanates whereof the melting point is greater than 10° C., preferably greater than 40° C., are suitable for this purpose. The stabilisers utilised for generating the retarding or surface-deactivating polymer cover which surrounds the isocyanate particles are mono- or polyfunctional amine stabilisers having primary and/or secondary amine groups. The dispersions which are described are used as cross-linking agents.
According to EP-A 0 467 168, aqueous preparations of copolymer dispersions and finely particulate surface-deactivated polyisocyanate solid suspensions are recommended. They are used as coating compositions for woven and nonwoven substrates. Compounds which convert the isocyanate groups located on the surface to urea or polyurea structures, such as, for example, water or primary as well as secondary amines, are described as deactivating agents. The coatings produced with these preparations, which are also designated one-component latently reactive layers, are cross-linked in heat simultaneously with drying.
EP-A 0 922 720 describes processes for the production and use of storage-stable latently reactive layers or powders of surface-deactivated solid polyisocyanates and dispersion polymers having functional groups. For this purpose aqueous dispersions are used which comprise a solid polyisocyanate, for example TDI dimer (2,4-diisocyanatotoluene dimerised by way of a uretdione group; Acima AG, Switzerland; TSE Industries, Clearwater, Fla., USA) which is deactivated at the surface, and a polymer which is reactive with isocyanate. The latently reactive layers or powders, which are storage-stable at room temperature, are cross-linked by heating above an activation temperature. The production of the polyisocyanate dispersion and the surface deactivation take place in accordance with EP-A 0 204 970. In order to control the surface deactivation and the cross-linking reaction the dispersions used may also comprise catalysts, preferably those which are stable to hydrolysis in the aqueous dispersion and which subsequently accelerate the heat-activated cross-linking reaction. Examples are organic compounds of tin, of iron, of lead, of cobalt, of bismuth, of antimony, of zinc. Alkylmercaptide compounds of dibutyltin are preferred on account of the greater stability of the catalyst to hydrolysis. Tertiary amines such as dimethylbenzylamine, diazabicycloundecene, as well as non-volatile polyurethane foam catalysts based on tertiary amines may also be used for specific purposes or in combination with metal catalysts.
EP-A 1 013 690 describes aqueous dispersions of isocyanate-reactive polymers and solid surface-deactivated polyisocyanates, with the polymers which are water-soluble or dispersed in water having a minimum film-forming temperature of <+5° C., a glass transition temperature of <−5° C. and a storage modulus at 10 Hertz and +10° C. of 107. The cross-linking reaction proceeds spontaneously under standard conditions—without heat activation—after evaporation of the water. According to DE-A 10 140 206 the mono- or polyamines used for the deactivation may have ionic groups, for example carboxylate or sulfonate groups, as a molecular structure constituent.
Particularly suitable latently reactive layers are distinguished in that they soften or decrystallise by brief heating, of less than 60 seconds' duration, to temperatures above 40° C., preferably to a temperature of 60° C. to 110° C., and in this state can be bonded with a substrate within the meaning of a pressure-sensitive or contact bonding. Simultaneously, under these conditions the surface-deactivated oligomeric solid isocyanate is activated which cross-links the polymers having isocyanate-reactive groups, which are comprised in the adhesive layer, and binds these to the substrate in the event that this also has isocyanate-reactive groups. This cross-linking process may proceed spontaneously, or optionally over a period of several days.
Preparations produced in accordance with the prior art which comprise as latently reactive cross-linking agents uretdiones of monomeric isocyanates involve the danger that significant quantities of the volatile isocyanate monomers may be liberated during processing or also during use of the finished components at elevated temperature as a result of cleavage of the thermally labile uretdione structure. This risk arises above all when substrates which must by their nature be subjected to thermal stress are processed. These include, for example, thermoplastic decorative foils which are applied by the thermoforming method, with heating beyond their softening point, to profiled furniture faces coated with one-component latently reactive systems. Automotive interior fitting elements may also be exposed to very high temperatures when the vehicle heats up in the sun. The isocyanate monomers may then outgas and pollute the atmosphere in the vehicle.
The object of the present invention was therefore to provide adhesive systems with which latently reactive layers can be created in which no liberation of isocyanate group-containing monomers occurs either during joining of the substrates which are to be bonded or during use of the same at elevated temperature.